Gene encoding a multidrug-resistance human P-glycoprotein homologue on chromosome 7p15-2 and uses thereof

ABSTRACT

The invention relates to an MDR family P-glycoprotein located on human chromosome 7p15-21, polynucleotide sequences encoding this P-glycoprotein and fragments thereof. This gene is utilized in methods for assessing cancer cell susceptibility to therapies directed against multidrug resistance, and for the design of diagnostic and therapeutic methods relating to cancer multidrug resistance. The invention also relates to methods for determining whether a test compound may inhibit multidrug resistance.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. Ser. No. 09/873,409, which was filed on Jun. 5, 2001. now U.S. Pat No. 6,846,883. The '409 application claims the benefit of U.S. provisional application No. 60/208,913, filed on Jun. 5, 2000.

FIELD OF THE INVENTION

The invention relates to genetic sequences encoding proteins which exhibit structural and functional features characteristic of P-glycoprotein family members associated with cancer multidrug resistance, immune regulatory functions, and unique functions in human pluripotent stem cells and other tissue progenitor cells. The invention encompasses substantially pure proteins, therapeutic treatments and diagnostic uses related to these proteins.

BACKGROUND OF THE INVENTION

P-glycoprotein, an adenosine triphosphate (ATP)-dependent drug efflux pump, is overexpressed in multidrug-resistant (MDR) tumor cells. It reduces the intracellular concentration of cytotoxic xenobiotics, thereby decreasing the effectiveness of many cancer chemotherapeutic regimens. P-glycoprotein belongs to the ABC (ATP-binding cassette) superfamily of active transporters, and is encoded by a multigene family in higher eukaryotes. Mammalian P-glycoprotein family members can be divided into three classes. Class I and class II P-glycoproteins confer multidrug resistance whereas class III proteins do not.

In humans, P-glycoprotein is encoded by two linked genes (“MDR1” and “MDR3”) on chromosome 7q21.1. MDR3 functions as a lipid translocase and mutations in this gene are associated with familial intrahepatic cholestasis. MDR1 confers drug resistance on certain cancer cells. In addition to being overexpressed in cancer cells, MDR1 P-glycoprotein is widely expressed in normal, predominantly secretory and absorptive human tissues, where it functions in diverse physiologic processes including cellular differentiation, cell proliferation and cell survival. In these normal cell types, P-glycoprotein functions in the transmembrane release or uptake of xenobiotics and certain therapeutic drugs, small peptide molecules, certain steroid compounds, and phospholipids.

P-glycoprotein is also expressed by lymphoid cell populations from human bone marrow and the peripheral blood. Specifically, P-glycoprotein has been shown to be expressed on the membrane of pluripotent stem cells, monocytes, dendritic cells, CD4+ and CD8+ T lymphocytes, natural killer cells, and B lymphocytes. In immune cells, P-glycoprotein functions in the transport of cytokines and other small molecules, which are critical for physiologic immune responses to occur. Specific blockade of P-glycoprotein can suppress the immune response to alloantigen and nominal antigen. However, a degree of redundancy exists for P-glycoprotein function in these cell types, pointing to the existence of additional, hitherto unidentified related molecules.

Pluripotent stem cells and other tissue progenitor cells also possess a unique P-glycoprotein-like activity, characterized by decreased intracellular accumulation of fluorescent dyes, which allows for the specific isolation of these cell types for therapeutic uses. However, it is thought that this function is not mediated by MDR1 P-glycoprotein, but rather by a related, as yet unidentified, P-glycoprotein family member.

Despite the irrefutable role of MDR1 P-glycoprotein in cancer multidrug resistance, attempts to improve chemotherapy by inhibiting this protein have met with only limited success. Thus, it may be inferred that there are homologous proteins that, like MDR1, are able to make cells resistant to therapeutic agents. In addition, it may be inferred that MDR1 homologous proteins serve P-glycoprotein-like functions in physiologic human tissues, in particular in cells of the immune system, plurnpotent stem cells and tissue progenitor cells, where either redundancy exists for MDR1 P-glycoprotein function, or where MDR1 P-glycoprotein is known to not promote the observed P-glycoprotein-associated activity.

SUMMARY OF THE INVENTION

The invention is directed to a new member of the human P-glycoprotein family of genes located on chromosome 7p15-2, encoding proteins which confer the multidrug resistant phenotype to tumor cells and/or serve critical physiologic functions in normal human tissues.

An examination of the structure of the new gene indicates that it encodes two semiautonomous homologous halves, each with their own transmembrane and ATP-binding domains. By alternative splicing and differential gene expression and/or posttranscriptional and posttranslational modifications, the new P-glycoprotein gene can encode several distinct P-glycoproteins:

-   -   The protein of SEQ ID NO:1 (amino acids 1-659) is encoded by 14         exons (SEQ ID NO:9) of human genomic DNA from clone AC005060 on         chromosome 7p15-21 and is made up of 5 transmembrane domains and         one ATP-binding domain.     -   The protein of SEQ ID NO:2 (amino acids 1-812) is encoded by 19         exons (SEQ ID NO:10) of human genomic DNA from the contiguous         clones AC002486 and AC005060 (AC002486 is the clone sequenced to         the left of clone AC005060) on chromosome 7p15-21 and is made up         of 5 transmembrane domains and two ATP-binding domains, of which         the first is located on the N-terminal side of transmembrane         domain #1, and the second on the C-terminal side of         transmembrane domain #5 of the protein, on the opposite side of         the plasma membrane. The protein of SEQ ID NO:2 can also be         expressed as a result of transsplicing of the mRNA (SEQ ID         NO: 9) encoding the protein of SEQ ID NO:1 and mRNA (SEQ ID         NO:11) encoding the protein of SEQ ID NO:3 described hereafter.         In addition, the protein of SEQ ID NO:2 may be expressed as a         result of posttranslational processing of the proteins of SEQ ID         NO:1 and NO:3.     -   The protein of SEQ ID NO:3 (amino acids 1-131) is encoded by 6         exons (SEQ ID NO:11) of human genomic DNA from clone AC002486 on         chromosome 7p15-21 and is made up of one ATP-binding domain and         no transmembrane domains.     -   The protein of SEQ ID NO:4 (amino acids 1-1058) is encoded by 20         exons (SEQ ID NO:12) of human genomic DNA from the contiguous         clones AC002486 and AC005060 on chromosome 7p15-21 and is made         up of 8 transmembrane domains and two ATP-binding domains, of         which the first is located between transmembrane domains #3 and         #4, and the second on the C-terminal side of transmembrane         domains #8, on the opposite side of the plasma membrane.     -   The protein of SEQ ID NO:5 (amino acids 1-1222) is encoded by 23         exons (SEQ ID NO:13) of human genomic DNA from the contiguous         clones AC002486 and AC005060 on chromosome 7p15-21 and is made         up of 12 transmembrane domains and two ATP-binding domains, of         which the first is located between transmembrane domains #7 and         #8, and the second on the C-terminal side of transmembrane         domain #12, on the opposite side of the plasma membrane.     -   The protein of SEQ ID NO:6 (amino acids 1-1195) is encoded by 24         exons (SEQ ID NO:14) of human genomic DNA from the contiguous         clones AC002486 and AC005060 on chromosome 7p15-21 and is made         up of 11 transmembrane domains and two ATP-binding domains, of         which the first is located between transmembrane domains #6 and         #7, and the second on the C-terminal side of transmembrane         domain #11, on the opposite side of the plasma membrane.     -   The protein of SEQ ID NO:7 (amino acids 1-541) is encoded by 10         exons (SEQ ID NO:15). of human genomic DNA from clone AC002486         on chromosome 7p15-21 and is made up of 7 transmembrane domains         and one ATP-binding domain on the C-terminal side of         transmembrane domain #7.     -   The protein of SEQ ID NO:8 (amino acids 1-514) is encoded by 11         exons (SEQ ID NO:16) of human genomic DNA from clone AC002486 on         chromosome 7p15-21 and is made up of 6 transmembrane domains and         one ATP-binding domain on the C-terrninal side of transmembrane         domain #6.

Cancer multidrug resistance may result from the expression of any of the proteins of SEQ ID NO:1, NO:2, NO:3, NO:4, NO:5, NO:6 NO:7 and NO:8. The proteins encoded by the 7p15-21 P-glycoprotein gene of the present invention may be used as markers for identifying cells likely to display multidrug resistance and can serve as targets in the design of new therapies for cancer patients. It will be understood that, except as otherwise indicated, reference to the P-glycoprotein of the present invention also includes any of the proteins of SEQ ID NO:1, NO:2, NO:3, NO:4, NO:5, NO:6, NO:7 and NO:8 as well.

The 7p15-21 P-glycoprotein confers chemoresistance to multiple chemotherapeutic agents, including cisplatinum, by mediating cellular drug efflux. Hence, specific blockade of this efflux fluction, for example by means of specific monoclonal antibody inhibition, can enhance intracellular drug accumulation and, as a result, drug toxicity and tumor cell killing. In addition, since 7p15-21 P-glycoprotein is functional in tumor cell proliferation, tumor growth can be therapeutically inhibited by administration of blocking specific monoclonal antibodies, even in the absence of concurrent chemotherapeutic agents. Among the proteins encoded by the 7p15-21 P-glycoprotein gene, the proteins of SEQ ID NO:1, NO:2, NO:3, NO:4, NO:5 and NO:6 are distinct from the proteins of SEQ ID NO:7 and NO:8 in that they are selectively expressed in certain cancer cells but not in non-cancerous normal tissues. Furthermore, the proteins of SEQ ID NO:1, NO:2, NO:3, NO:4, NO:5 and NO:6 are expressed preferentially in those cancers which exhibit the highest degrees of chemoresistance to chemotherapeutic drugs, such as for example human malignant melanoma. Because of their selective expression in certain cancers but not in normal tissues, the proteins of SEQ ID NO:1, NO:2, NO:3, NO:4, NO:5 and NO:6 can be therapeutically targeted not only via inhibition of cytotoxic drug efflux or inhibition of tumor proliferation by specific monoclonal antibodies, but also by additional means, including tumor-specific cell killing mediated by cell toxin-conjugated specific monoclonal antibodies, or by therapeutic administration to afflicted patients of tumor antigen-specific vaccine preparations.

The proteins of SEQ ID NO:7 and NO:8 encoded by the 7p15-21 gene can also be expressed in certain non-cancerous normal human tissues. The invention thus provides for additional uses as relating to the function of these select proteins in physiologic tissues. Among those normal tissues, the proteins of SEQ ID NO:7 and SEQ ID NO:8 are preferentially expressed at high levels in pluripotent stem cells and other tissue progenitor cells, where they function in the transmembrane transport of xenobiotics and other small molecules. The invention provides thus for means to specifically detect and enrich these stem cells and progenitor cells from cell mixtures and preparations in which they are contained, by detection of the cells with labeled specific monoclonal antibodies.

The proteins of SEQ ID NO:7 and NO:8 are also expressed to a certain degree in most other normal human tissues, including in cells of the immune system such as T cells, monocytes and differentiated antigen presenting cells, where they function in the efflux of cytokines and the uptake of small molecules including peptides and antigen, thus serving a critical role for the integrity of normal immune responses. When these functions are inhibited, for example by specific monoclonal antibody blockade, the normal immune response can be modulated, which can be utilized in the prevention and/or the therapy of allograft rejection in clinical organ transplantation, and also in various autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. In addition, when expressed in human immune cells and other human tissues such as the endothelium of the blood-brain barrier and the epithelia of the gastrointestinal tract and the kidney, blockade of the protein can furthermore be therapeutically employed to selectively alter the uptake and secretion, and hence the pharmacological distribution, pharmacokinetics and therapeutic efficacy of those exogenously administered therapeutic drugs which are substrates of said proteins.

In a first aspect, the invention is directed to substantially pure proteins consisting essentially of the amino acid sequence of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8. The term “consisting essentially of” is meant to encompass proteins having exactly the same amino acid sequences, as well as proteins with insubstantially different sequences, as evidenced by their possessing the same basic functional properties. A “substantially purified” isoform is one that has been separated from other accompanying biological components and will typically comprise at least 85% of a sample, with greater percentages being preferred. Many means are available for assessing the purity of a protein within a sample, including analysis by polyacrylamide gel electrophoresis, chromatography and analytical centrifugation. A preferred method for assessing purity is by Western blotting using an antibody directed against epitopes of the 7p15-21 P-glycoprotein of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8. The invention also encompasses “MDR peptides” which are defined herein as consisting of a sequence element of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8. of at least 10 and preferably at least 15 or 20 residues. These may be used in the generation of antibodies. It is stipulated that an MDR peptide cannot have a sequence that is the same as any set of 10 to 15 contiguous residues in the sequence LSGGQKQRIAIARAL (SEQ ID NO:17). These proteins and MDR peptides may also be administered therapeutically to cancer patients afflicted with 7p15-21 P-glycoprotein expressing tumors, as a tumor vaccine to elicit an endogenous immune response directed against these tumors, to result in tumor-specific cell killing

In another embodiment, the invention is directed to an antibody made by a process comprising the step of administering to an animal host a protein encoded by SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8, or an MDR peptide as described above. The protein or peptide should be administered to the animal at a dosage sufficient to induce antibody formation. Antibodies may be monoclonal or polyclonal. In the latter case, antibodies are preferably produced by injecting a pharmaceutically acceptable preparation into a mouse, followed by fusing mouse spleen cells with myeloma cells using techniques known in the art. The antibodies obtained should bind selectively to the proteins of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8. Selective binding, in this context, means that an antibody has at least a 100-fold greater affinity for one or more of these proteins than for any other protein normally found in human cells.

The invention is also directed to a substantially pure polynucleotide consisting essentially of a nucleotide sequence encoding the proteins of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8, or an MDR peptide. Preferably, the polynucleotide consists essentially of the nucleotide sequence of SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15 or SEQ ID NO:16. The invention includes expression vectors comprising a distinct coding element consisting of these polynucleotides; and host cells transformed with such vectors. A “distinct coding element” refers to the portion of an expression vector responsible for determining the amino acid sequence of an expressed protein. The invention comprises all such elements producing proteins corresponding to the amino acid sequences shown in SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8, as well as other proteins having substantially the same structure and function.

The invention includes recombinant protein made by host cells transformed by an expression vector as discussed above. The recombinant protein may be isolated using standard techniques, including affinity chromatography with antibodies against epitopes of 7p15-21 P-glycoprotein. Preferably, the polynucleotide used in vectors for expressing such a recombinant P-glycoprotein consists essentially of the nucleotide sequences of SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15 or SEQ ID NO:16. Oligonucleotides complementary to SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15 or SEQ ID NO:16 and at least 15 nucleotides in length may be used as antisense inhibitors. These may be administered to patients undergoing cancer chemotherapy to increase the effectiveness of cytotoxic drugs. The in vivo transfection of cells has been known for many years and may be accomplished using viral vectors (see e.g., U.S. Pat. No. 6,020,191); liposomes (see e.g., Nicolau, Meth. Enzymol 149:157-176 (1987)); DNA complexed to agents that facilitate cellular uptake (see e.g., U.S. Pat. No. 5,264,618; WO 98/14431); or even by simply injecting naked DNA. (see e.g., U.S. Pat. No. 5,693,622). Any of these procedures may be used to deliver the antisense oiligonucleotides of the present invention.

The invention is also directed to a method for determining whether a cancer cell will respond to therapies aimed at reversing multidrug resistance by measuring the expression of the genes encoding the proteins of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8. This method may be used to detect the existence of the multidrug resistant phenotype in cancer cells or to track the development of multidrug resistance over time by monitoring changes in gene expression in cultured cells.

In another embodiment, the invention provides for a method of determining whether a test compound inhibits multidrug resistance in cells caused by a gene encoding proteins of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8. This method comprises expressing a gene encoding one or more of these polypeptides in cells that are otherwise not multidrug resistant and exposing these cells to one or more cytotoxic drugs in the presence of a test compound. Cellular survival is measured after exposure and the results obtained are compared with those from incubations carried out in essentially the same manner but in the absence of the test compound. It is concluded that the test compound inhibits multidrug resistance if cellular survival is decreased to a significant extent in incubations carried out in the presence of the test compound relative to that seen in its absence.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a novel member of the P-glycoprotein family of drug resistance related proteins, to genetic sequences encoding this protein, to methods of determining whether a cancer cell will respond to therapies aimed at reversing P-glycoprotein mediated drug resistance, and to a method of screening test compounds for their ability to inhibit multidrug resistance. The novel P-glycoprotein gene can encode the proteins of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8.

It will be understood that the invention encompasses not only sequences identical to those shown but also sequences that are essentially the same as evidenced by their retaining the same basic structural and functional characteristics. For example, techniques such as site directed mutagenesis may be used to introduce variations into a protein's structure. Variations in P-glycoprotein introduced by this or other similar methods are encompassed by the invention provided that the resulting protein retains its basic biological properties, particularly with respect to the inducement of multidrug resistance in mammalian cells.

DNA sequences encoding the proteins of the invention may be obtained from any tissue or cellular source in which they are expressed. For example, cultured cell lines may be engineered to express the P-glycoprotein gene using recombinant techniques or by continuous exposure to chemotherapeutic agents. Alternatively, sequences may be isolated from primary cells obtained from tumors.

Many methods are available for isolating DNA sequences and may be adapted for the isolation of the chromosome 7p15-21 (hereinafter “chromosome 7p”) P-glycoprotein gene (see, e.g., Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Press (1989)). For example, one method is to screen a cDNA library that has been prepared by reverse transcribing mRNA isolated from tissues or cells that express the gene. The library may be prepared from, for example, human melanocyte or testis tissue and probes for screening may be synthesized based upon the sequences shown in the Sequence Listing. The probes are preferably at least 14 nucleotides long and are optimally selected from a region believed to be unique to the chromosome 7 p P-glycoprotein gene.

As an alternative, amplification of a desired sequence may be achieved by the polymerase chain reaction (“PCR”) of reverse transcribed RNA. Primers for PCR may be constructed using the sequences shown in SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15 or SEQ ID NO:16, and confirmation of the presence of chromosome 7p P-glycoprotein cDNA may be obtained by the sequencing of amplification products.

Expression of recombinant protein may be induced in a host cell by transforming it with an appropriate expression vector. The vector should contain transcriptional and translational signals recognizable by the host together with the desired structural sequence, preferably in double stranded form, in an operable linkage. For example, the P-glycoprotein DNA sequence should be positioned such that regulatory sequences present in the vector control the synthesis of mRNA and protein having the desired sequence is produced.

Preferably, nucleic acid encoding the P-glycoprotein of the invention is expressed in eukaryotic cells, especially mammalian cells. Such cells are capable of promoting post-translational modifications necessary to ensure that the recombinant protein is structurally and functionally the same as the protein isolated from, for example, multidrug resistant tumor cells. Examples of mammalian cells known to provide proper post-translational modification of cloned proteins include, inter alia, NIH-3T3 cells, CHO cells, HeLA cells, LM(tk-) cells, and the like. Eukaryotic promoters known to control recombinant gene expression are preferably utilized to drive transcription of chromosome 7p P-glycoprotein DNA, and may include that of the mouse metallothionein I gene, the TK promoter of Herpes virus, the CMV early promoter and the SV40 early promoter. Transcription may also be directed by prokaryotic promoters, such as those capable of recognizing T4 polymerase, the P_(R) and P_(L) promoters of bacteriophage lambda, and the trp, recA, heat shock and lacZ promoters of E. coli.

Expression vectors may be introduced into host cells by methods such as calcium phosphate precipitation, microinjection, electroporation or viral transfer and cells expressing the recombinant protein sequence can be selected by techniques known in the art. Confirmation of expression may be obtained by PCR amplification of P-glycoprotein sequences using primers selected from the sequences shown in SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15 or SEQ ID NO:16.

Recombinant protein may be purified using standard techniques well known in the art. Such techniques may include filtration, precipitation, chromatography and electrophoretic methods. Purity can be assessed by performing electrophoresis on a polyacrylamide gel and visualizing proteins using standard staining methodology. Western blotting also may be performed using an antibody to chromosome 7p P-glycoprotein.

The invention is also directed to antibodies raised against the chromosome 7p P-glycoprotein. The process for producing such antibodies may involve either injecting the 7p P-glycoprotein itself into an appropriate animal or injecting short antigenic peptides made to correspond to different regions of the protein. These peptides should be at least 5 amino acids in length and should, preferably, be selected from regions believed to be unique to the 7p P-glycoprotein. Methods for generating and detecting antibodies are well known in the art, and are taught by such references as: Harlow, et al., Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y. (1988); Klein, Immunology: The Science of Self-Nonself Discrimination, (1982); Kennett et al., Monoclonal Antibodies and Hybridomas: A New Dimension in Biological Analyses, (1980); and Campbell, “Monoclonal Antibody Technology”, in Laboratory Techniques in Biochemistry and Molecular Biology, (1984).

The term “antibody”, as used herein, is meant to include intact molecules as well as fragments that retain their ability to bind antigen, such as Fab and F(ab′)₂ fragments. The term “antibody” is also defined herein as relating to both monoclonal antibodies and polyclonal antibodies. Polyclonal antibodies are derived from the sera of animals immunized with a chromosome 7p P-glycoprotein antigen. Monoclonal antibodies to the protein can be prepared using hybridoma technology, as taught by such references as: Kohler, et al., Nature 256:495 (1975); and Hammerling, et al., in: Monoclonal Antibodies and T-Cell Hybridomas, Elsevier, N.Y., pp. 563-681 (1981). In general, this technology involves immunizing an immunocompetent animal, typically a mouse, with either intact chromosome 7p P-glycoprotein or a fragment derived therefrom. Splenocytes are then extracted from the immunized animal and are fused with suitable myeloma cells, such as SP₂O cells. Thereafter, the resulting hybridoma cells are selectively maintained in HAT medium and then cloned by limited dilution (Wands, et al., Gastroenterology 80:225-232 (1981)). Cells obtained through such selection are then assayed to identify clones which secrete antibodies capable of binding chromosome 7p P-glycoprotein.

Antibodies or fragments of antibodies of the invention may be used to detect the presence of chromosome 7p P-glycoprotein in any of a variety of immunoassays. For example, antibodies may be used in radioimmunoassays or in immunometric assays, also known as “two-site” or “sandwich” assays (see Chard, “An Introduction to Radioimmune Assay and related Techniques,” in: Laboratory Techniques in Biochemistry and Molecular Biology, North Holland Publishing Co., N.Y. (1978)). In a typical immunometric assay, a quantity of unlabeled antibody is bound to a solid support that is insoluble in the fluid being tested, such as blood, lymph, cellular extracts and the like. Following the initial binding of antigen to immobilized antibody, a quantity of detectably labeled second antibody (which may or may not be the same as the first) is added to permit detection and/or quantitation of bound antigen (see, e.g. Radioimmune Assay Method, Kirkham, et al., Ed. pp. 199-206, E&S Livingstone, Edinburgh (1970)). Many variations of these types of assays are known in the art and may be employed for the detection of 7p P-glycoprotein.

Antibodies to chromosome 7p P-glycoprotein may also be used in purification procedures (see generally, Dean et al., Affinity Chromatography, A Practical Approach, IRL Press (1986)). Typically, antibody is immobilized on a chromatographic matrix such as Sepharose, 4B. The matrix is then packed into a column and the preparation containing chromosome 7p P-glycoprotein is passed through under conditions that promote binding, e.g., low salt conditions. The column is then washed protein is eluted using buffer that promotes dissociation from antibody, e.g., buffer having an altered pH or salt concentration. The eluted protein may be transferred into a buffer, for example via dialysis, and thereafter either stored or used directly. Antibodies may also be used in Western blotting for the detection of chromosome 7p P-glycoprotein in a sample. For these types of assays, antibody may be used which has either been developed specifically to react with chromosome 7p P-glycoprotein or which reacts with an epitope of the protein.

The detection of the chromosome 7p P-glycoprotein may be used to determine whether tumor cells are multidrug resistant. Likewise, detection of changes in the expression of P-glycoprotein may be useful in predicting the development of multidrug resistance in cells. The cDNA of this P-glycoprotein may be useful in designing primers for diagnostic PCR, probe design for diagnostic Northern blotting, RNase protection assays, and for the design of antisense oligonucleotides complementary to the predicted cDNA for use in gene-targeting strategies for the reversal of multidrug resistance. Both in vitro and in vivo diagnostic and therapeutic uses for antisense nucleotide sequences to the chromosome 7p P-glycoprotein are envisioned.

The primary amino acid sequence and protein structure of the chromosome 7p P-glycoprotein may be utilized in the production of monoclonal antibodies (mAbs) that can be used in the diagnosis and therapy of multidrug resistant cancer. For example, synthetic peptides resembling native amino acid sequences from particular extracellular domains as determined by membrane topology prediction may be useful for developing inhibitory mAbs directed against extracellular epitopes of the chromosome 7p P-glycoprotein. Additionally, 10-20 mer synthetic peptide sequences derived from the primary amino acid sequence not included in the above-mentioned extracellular loop sequences may be useful in the development of specific diagnostic monoclonal antibodies. Specific mAbs may be employed in diagnostic FACS analysis, Western blotting, and immunohistochemistry. Such mAbs may also be employed for in vivo diagnostic uses, where label-conjugated mAbs can be used to assess tumor burden, tumor localization or residual tumor mass following chemotherapy or surgical therapy of 7p15-21 P-glycoprotein-expressing tumors.

Specific mAbs can also be used for therapeutic purposes in cancer patients. In particular, they may be administered to reverse cancer multidrug resistance in patients receiving chemotherapeutic agents that are substrates for 7p P-glycoprotein efflux, e.g., cisplatin. In addition, specific mAbs may be used therapeutically in cancer patients for tumor-specific cell killing, either administered in an unconjugated form, resulting in immune-mediated tumor killing, or in a cell toxin-conjugated form (for example conjugated to radioactive iodine or chemical toxins), resulting in direct tumor-specific cell killing.

Specific mAbs can also be used for therapeutic purposes other than cancer multidrug resistance. Based on the predicted immunoregulatory function of 7p P-glycoprotein, these mAbs can be given to patients to prevent and/or treat organ transplant rejection, and also diverse autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. Furthermore, since P-glycoproteins function in the uptake, excretion and tissue-specific distribution of a variety of pharmacological and chemical compounds, and have been implicated in mechanisms of oral bioavailability, blood/brain barrier function and renal, hepatic and biliary excretion mechanisms of several drugs, specific mAbs can be administered therapeutically to alter the pharmacokinetics and availability of those therapeutic drugs which are substrates for 7p P-glycoprotein-mediated transport function.

The compositions and methods of the present invention may have a number of uses in addition to those described above. For example, pluripotent stem cells and tissue progenitor cells such as hematopoietic stem cells, neuroprogenitor cells and muscle progenitor cells are known to possess P-glycoprotein-like efflux activities for small molecules and fluorescent dyes. Chromosome 7p P-glycoprotein may play a role in the transport of such substrates, and thus may serve as a marker for the isolation of such stem cells and progenitor cells via, for example, FACS analysis. Also, since MDR1 P-glycoprotein appears to be involved in cellular differentiation, cell proliferation, cell survival, and certain immune responses, chromosome 7p P-glycoprotein, due to its homology with MDR 1 P-glycoprotein, is expected to play a role in such physiological functions as well. Thus, chromosome 7p P-glycoprotein gene and protein sequences may be useful in modulating pathophysiological disruptions of these MDR-related functions.

EXAMPLES

Since new genomic sequence information is currently being produced at a rapid pace via the human genome project, databases containing such genomic information potentially contain sequences of heretofore unidentified members of the P-glycoprotein family. Mammalian P-glycoprotein family members share characteristic amino acid sequences and protein epitopes, and assume similar conformations. Thus, a protein homology-based search was conducted in an attempt to identify novel P-glycoprotein-encoding genes. Gene-analytic and protein-analytic bioinformatics tools were utilized to further characterize the nucleic acid sequence and predicted protein structure of identified candidate genes. Specifically, the National Center for Biotechnology Information (NCBI) tblastn application was used to compare conserved amino acid sequences derived from the known structure of the human MDR1 P-glycoprotein with the NCBI non-redundant homo sapiens nucleotide sequence database dynamically translated in all reading frames. The signature sequence common to members of the ABC transporter family, a 15 mer amino acid sequence LSGGQKQRIAIARAL (SEQ ID NO:17), was used to identify human genomic DNA sequences encoding homologous protein structures. Known hexamer amino acid sequences of three P-glycoprotein-specific monoclonal antibody (mAb)-binding epitopes were also employed.

Human genomic DNA clones identified in the manner described above were screened for vector contamination using the VecScreen program. Additionally, these clones were subjected to systematic homology mapping using overlapping contiguous 20-mer amino acid sequences derived from the human MDR1 protein structure and the tblastn search program. Candidate genomic DNA sequences encoding homologous amino acid sequences were compared to open reading frame (ORF) sequences predicted in each DNA clone using the NCBI ORF Finder program (Altschul, et al., Nucleic Acids Res. 25:3389-402 (1997)). Genomic ORFs containing homologous DNA sequences were then analyzed using the NetGene2 software package in order to predict intron splice sites in the candidate genes (Brunak et al., J Mol. Biol. 220:49-65 (1991)).

A cDNA sequence was generated by conceptual linear transcription of predicted adjacent DNA exon structures. Utilizing this approach, two adjacent overlapping human genomic clones, CTA-367017 (AC002486, 79611 base pairs in length) and CTB-86D3 (AC005060, 120169 base pairs in length, sequenced to the right) were identified as forming part of an unanchored island of unknown orientation on chromosome 7p15-21. These overlapping clones were found to contain a gene sequence encoding a novel member of the human P-glycoprotein family.

In order to determine whether the predicted gene structure was expressed in human tissues, the generated cDNA sequence was compared to the human NCBI dbest non-redundant expressed sequence tags (EST) database, as described by Altschul et al, and several ESTs complementary to predicted exons from the genomic clone AC002486 were identified. Polymerase chain reaction (PCR) primers were then designed based on available sequence information in the database at the National Center for Biotechnology Information (NCBI) and the bioinforrnatic analysis as described above. Using these gene-specific oligonucleotide primers and the PCR technique on reverse transcribed total messenger RNA (mRNA) isolated from several human cancer cell lines and normal human tissues, including the human G3361melanoma cell line, the MCF-7 breast carcinoma cell line, the SCC25 squamous cell carcinoma cell line, the U937 leukemia cell line, and normal peripheral blood mononuclear cells (PBMC), cDNA sequences derived from the novel 7p15-21 P-glycoprotein gene were amplified and the PCR products were subsequently sequenced using the dideoxy chain termination method on both strands.

The intron-exon structure of several gene products encoded by the 7p15-21 P-glycoprotein gene was determined by comparison of predicted and sequenced cDNA clones with genomic sequence information from the 7p15-21 P-glycoprotein gene locus (clones AC002486 and AC005060), as shown in SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15 and SEQ ID NO:16. Protein structures encoded by the new 7p15-21 gene were then generated by conceptual amino acid translation of the predicted oligonucleotide sequences of SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15 and SEQ ID NO:16, as shown in SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 and SEQ ID NO:8. These amino acid sequence were then compared with the NCBI non-redundant peptide sequence for sequence homology using the NCBI blastp program. The predicted amino acid sequences of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7 and SEQ ID NO:8. were also classified using the PIR-Intemational Protein Family Classification System (Barker, et al., Nucleic Acids Res. 28:41-4 (2000); Huang et al., Nucleic Acids Res. 28:273-6 (2000)). Potential flnctional characteristics of. the predicted proteins were determined by comparative analysis of the primary amino acid composition as well as by using the TMHMM1.0 software package for the prediction of transmembrane helix formation in mammalian proteins (Sonnhammer et al., Ismb 6:175-82 (1998)).

The novel 7p15-21 P-glycoprotein gene can encode several distinct P-glycoprotein isoforms which display 68% sequence homology with both human MDR1 and MDR3. A similar degree of homology was found with respective mouse and hamster isoforms of these human genes. Primary amino acid sequence analysis suggests that the chromosome 7p15-21 P-glycoprotein may express the C32 and anti-P-glycoprotein mAb binding epitope, but not the C219 epitope conserved in all other known P-glycoprotein isoforms (Georges, et al., Proc. Natl Acad Sci USA 87:152-6. (1990)).

Structural prediction revealed that the 7p15-21 P-glycoprotein gene encodes P-glycoprotein isoforms which exhibit structural similarities but also distinctive differences compared to known members of the P-glycoprotein family, as disclosed by Georges et al. For example, the protein of SEQ ID NO:2 contains two ATP-binding domains which are located on opposite sides of the plasma membrane, providing for a unique extracellular ATP-binding domain which is predicted to bind extracellular ATP. Based on these distinctive differences, it is predicted that 7p15-21 P-glycoprotein is not only involved in small molecule efflux, but that some of its isoforms are also functional in the energy-dependent uptake of small molecules. The PIR classification system confirmed the discovered chromosome 7p15-21 P-glycoprotein to be a member of the family of multidrug resistance proteins and the family of ATP-binding cassette homology superfamilies.

PCR analysis using gene-specific primers demonstrated that cDNA encoding the proteins of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5 and SEQ ID NO:6, which in each case involves exons encoded on genomic clone AC005060, was preferentially expressed in human melanoma cells but not most other cancers tested, unlike cDNAs encoding the proteins of SEQ ID NO:7 and SEQ ID NO:8, which was found expressed in most cancers examined and also physiologic human tissues. This emphasizes that a subset of 7p15-21 P-glycoprotein gene products can be selectively targeted in certain cancers that display particularly high degrees of chemoresistance, such as human melanoma.

To assess the expression and finction of 7p15-21 P-glycoprotein and the effect of specific modulation on transport function and chemoresistance, polyclonal antibodies were raised against the MDR peptides CGTSLILNGEPGYTI (SEQ ID NO:18) and RFGAYLIQAGRMTPEGC (SEQ ID NO:19), corresponding to distinct extracellular loop epitopes of 7p15-21 P-glycoprotein, by injecting mice with these antigenic peptides conjugated to the carrier substance KLH. To assess 7p15-21 P-glycoprotein surface expression of human tumor cells, indirect surface immunostaining and single color flow cytometry of freshly harvested cells was performed. To assess the effects of 7p15-21 P-gp inhibition on P-gp-mediated fluorescent dye efflux, tumor cells were incubated with anti-7p15-21 P-glycoprotein polyclonal Ab followed by addition of calcein-AM and subsequent serial cell fluorescence measurements by flow cytometry.

These studies demonstrated that P-glycoprotein is expressed on tumor cells, and that the RFGAYLIQAGRMTPEGC (SEQ ID NO:19) epitope contained in the proteins of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5 and SEQ ID NO:6, is preferentially expressed in human melanoma at high levels, whereas the CGTSLILNGEPGYTI (SEQ ID NO:18) epitope, also contained in SEQ ID NO:7 and SEQ ID NO:8, is also expressed in other types of cancer and normal human cells. Antibodies against the CGTSLILNGEPGYTI (SEQ ID NO:18) epitope inhibited both dye uptake and also dye efflux dependent on cell type, indicating a dual function of the various gene products of 7p15-21 P-glycoprotein in these distinct processes. These antibodies also enhanced cell cytotoxicity of cisplatinum in specific cell killing assays in melanoma and also breast cancer among others, indicative of their potential therapeutic usefulness in the treatment of cancer patients.

Certain cancers are known to exhibit chromosomal rearrangement in the 7p15-21 region, and such mutations can be associated with the emergence of the MDR phenotype. This raises the possibility that gene rearrangement in these cancers potentially results form episome and double minute (DM) chromosome formation during the process of gene amplification of 7p15-21 P-glycoprotein under mutagenic stresses such as chemotherapy. Cells expressing MDR1-mediated multidrug resistance are known to undergo such chromosomal rearrangements and DM chromosome formation (Scehoenlein et al., Mol. Biol. Cell 3:507-20 (1992); Mickley et al., J. Clin. Invest. 99:1947-57 (1997); Knutsen et al., Genes Chromosomes Cancer 23:44-54 (1998)). Thus, the chromosome 7p15-21 P-glycoprotein gene products of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5 and SEQ ID NO:6 may be selectively overexpressed in certain cancer cells, thereby contributing to the acquired drug resistance of such cancer cells while remaining silent in normal cells. This differential expression pattern may be employed in the detection and reversal of multidrug resistance of tumorigenic mammalian cells. 

1. A method of determining whether a cancer cell expresses a gene that contributes to multidrug resistance, comprising selecting a cancer cell and assaying said cancer cell for the expression of the protein of SEQ ID NO:2 and associating the expression of the protein of SEQ ID NO:2 with the cancer cell having multidrug resistance.
 2. the method of claim 1, wherein expression of said protein is determined using an antibody that preferentially binds to a protein with the sequence of SEQ ID NO:2.
 3. The method of claim 2, wherein said antibody is a polyclonal antibody.
 4. The method of claim 2, wherein said antibody is a monoclonal antibody.
 5. The method of claim 1, wherein expression of said protein is determined using an immunoassay.
 6. The method of claim 5, wherein said immunoassay is a radioimmunoassay.
 7. The method of claim 5, wherein said immunoassay is an immunometric assay.
 8. The method of any one of claims 2-7, wherein said cancer cell is a melanoma cell. 